Transglutaminase is an enzyme of unknown function in the liver. This enzyme, however, can couple (covalently) in vitro selected amines such as phenethylamine, isoniazid, and putrescine to certain endogenous proteins. Calcium is a requisite for this reaction. This enzyme activity appears operative in plasma as Factor XIII for the catalysis of the epsilon(gamma-glutamyl) lysine bonds between fibrin monomers. Factor XIII prior to the sequential clotting reactions, exists--unlike the liver enzyme--as a zymogen in plasma. There is some evidence to indicate that the hepatic enzyme might function in vivo in the manner of Factor XIII by catalyzing epsilon(gamma-glutamyl) lysine bonds for membrane proteins (P.J. Birkbichler et al. Biochim. Biophys. Acta 291: 149, 1973). The objectives of the present program are to: (1) purify and to characterize rabbit liver transglutaminase, (2) determine what proteins in the cytosol and membrane fractions of the hepatocyte can act as acceptors for amines such as putrescine, spermine, isoniazid and tyramine, (3) determine any role of Ca2 ions or other divalent metals in the stimulation of hepatic intracellular transglutaminase. Unlike the blood enzyme (Factor XIII), hepatic transglutaminase needs no proteolysis to initiate activity. However, it is difficult to explain how transglutaminase is operative in vivo when intracellular calcium is so very low in concentration (approximately 1 x 10 to the minus sixth power M), and (4) to identify any intracellular proteins which are cross-linked by hepatic transglutaminase and to examine any structural or dynamic roles of these proteins.